Production of low sulfur iron



Oct. 6, 1964 w. HALLEY 3,151,973

PRODUCTION OF LOW SULFUR IRON Filed Feb. 23, 1962 OFF J3 J0 IRON REDUCTION z/il l REDUCED /14 g ORE COAL

17 C15 16 COAL g HEARTH 1 j SLAG J ,2! L MOLTEN Ji COMBUSTION IRON eAsEs i IGASIFIER uLsAroRf ag 26 PULSATOR HEAT J 44 r EXCHANGER a? I SULFUR VYQTER 9 r2? Q REMOVAL v 25 l HEAT SCRUBBER EXCHANGE)? 2 WATER 29 26 WATER m f2? 32 OUT I 29 r COMPRESSOR SCRUBBER 31 37 WATER '34 /41 22 ou'r 96 v su l. =u R COMPRESSOR REMOVAL 1 J INVENTOR.

United States Patent ()fi ice 3,151,973 Patented Get. 6, 1964 3,151,973 PRQBUGHON F LQW SULFUR IRGN ames W. Halley, Dune Acres, ind, assignor t inland Steel Company, Chicago, EL, a corporation of Delaware Filed Feb. 23, B62, Ser. No. 174,957 13 Elaims. (Cl. 7538) This invention relates to improvements in the direct reduction of iron oxide ore to obtain molten iron and, more particularly, to a method of obtaining a low sulfur content in the molten iron so produced.

In my prior US. Patent No. 2,919,983, issued January 5, 1960, I have described a so-called balanced proc ess for the direct reduction of iron oxide ore to obtain molten iron wherein the chemical and thermal requirements of the process are supplied with a minimum consumption of fuel and oxygen. Briefly described, the process of the aforementioned patent involves three principal process stages: (1) a combined melting and gas generator zone, also referred to as a hearth zone, wherein a fuel is burned with oxygen in order to melt and separate the iron from the reduced ore and at the same time producing combustion gas, (2) a reforming zone wherein the combustion gas from the hearth zone is treated with additional fuel and oxygen to increase the reducing capacity of the as, and (3) a reduction zone wherein subdivided iron ore is contacted with the reformed gas from the reforming zone and from which the resultant reduced iron ore is passed to the hearth zone. The balanced operation of this process is achieved by utilizing a certain critical combination of operating conditions and procedures in the several steps.

In general, pulverized coal is the preferred fuel to be burned in the hearth zone and in the reforming zone of the above-mentioned process. However, I have now found that nearly all of the sulfur content of the coal will accumulate in the molten iron product unless special precautions are taken to prevent such sulfur contamination. it would be possible to maintain sulfur at a low level in the molten iron by adding a sufiicient quantity of a basic material, such as calcium oxide or magnesium oxide, to the hearth zone in order to produce a highly basic slag which will absorb all of the sulfur and prevent it from entering the molten iron. However, the addition of large quantities of basic material to the hearth will have an adverse affect on the thermal balance of the process because of the additional heat required to melt the added basic material which Would thereby increase the fuel and oxygen consumption of the process.

Accordingly, a primary object of the present invention is to provide a novel and improved means for producing molten iron of low sulfur content in an integrated iron oxide ore reduction and melting process.

Another object of the invention is to provide a novel and improved method of preventing sulfur accumulation in the molten iron product of a reduction process of the aforementioned type without adversely afiecting the thermal or chemical balance of the process.

A further object of the invention is to provide, in an integrated reduction and melting process of the type described, a novel and improved means for removing the sulfur introduced into the system with the fuel without increasing the thermal burden of the process or the fuel and oxygen consumption.

Other objects and advantages of the invention will become apparent from the subsequent detailed description taken in conjunction with the accompanying drawing, wherein:

FIG. 1 is a block flow diagram illustrating one specific embodiment of the invention; and

FIG. 2 is a portion of a similar flow diagram illustrat ing a modification of the process of FIG. 1.

I have found that if an acid slag is maintained in the melting or hearth zone of the process, the molten iron will absorb very little sulfur from the combustion gases in the hearth zone. However, this step alone does not eliminate the sulfur problem because the sulfur is carried by the combustion gases which, after reforming to increase their reducing capacity, are introduced into the reduction zone. It Wfll be evident that the sulfur content of the reducing gas would be absorbed by the iron oxide ore during the reduction step. As a result, the reduced or partially reduced iron ore solids leaving the reduction zone and entering the hearth zone would have a high sulfur content so that the molten iron would be high in sulfur. Accordingly, in addition to maintaining an acid slag in the hearth zone to avoid absorption of sulfur from the combination gases by the molten iron, the present invention also comprises the step of removing sulfur from the reducing gas before it is introduced into the reduction zone. Thus, by maintaining an acid slag in the hearth zone and by subsequently treating the reducing gas to remove sulf r therefrom before the gas is introduced into the reduction zone, I am able to produce a low sulfur product in the reduction zone and prevent resulfurization of the molten iron in the hearth zone.

in FIG. 1, subdivided or granular iron oxide ore is introduced through a line 10 to a reduction zone 11 and is therein contacted with CO-rich reducing gases introduced to the zone 11 through a line 12.

The ores which may be used in the process comprise any of the well-known iron oxide ores including hematite, magnetite, and others which may contain at least about 5 wt. percent and as much as 45 wt. percent gangue materials, particularly silica and alumina. Other ores similar to iron ore such as iron-manganese ores may also be used. It is also within the scope of the invention to charge to the reduction zone 11 other iron oxide materials such as mill scale, etc. In the reduction zone 11 the well-known reduction reactions of iron oxide with CO (and H take place with the heat of reaction being supplied by the sensible heat or" the reducing gases which are at a temperature of from about 900 F. to about 1800 F., as hereinafter escribed. Efiiuent reducing gases of depleted CO content are discharged through a line 13 from the reduction zone ill.

The resultant ore particles comprising reduced iron, gangue, and a certain amount of unreduced iron oxide pass from the reduction zone ll through a line 14 to a hearth zone it; which comprises a combined melting and gas generator zone. A solid carbonaceous fuel is introduced in admixture with a high oxygen content gas through a line 17 and combustion of the fuel with the oxygen takes place witllin the hearth zone 16. The preferred solid carbonaceous fuel burned in the hearth zone 16 is coal such as anthracite, bituminous or sub-bituminous coal in pulverized form. The oxygen-rich gas introduced with the fuel should contain at least oxygen, e.g., a commercial grade of straight oxygen which may be 98-99% pure, or oxygen enriched air.

As a result of the high temperature obtained in the hearth zone 16, the iron content of the reduced ore solids is melted and a fluid slag is also formed from the gangue constituents of the ore. Molten iron and slag may be tapped from the hearth as desired through lines 13 and 19, respectively. The fluid slag formed in the hearth zone acts as a protective layer or blanket overlying the molten iron so as to protect the latter from reoxidation. Coal or other carbonaceous solid is also supplied separately to the hearth zone in through a line 15 for the purpose of completing the reduction of the unreduced iron oxide introduced with the solids in line 14. The added carbon also protects the molten iron from reoxidation and effects carburization of the molten iron to any desired extent.

Since the combustion gases produced in the hearth zone 16 contain a substantial amount of CO they must be subjected to an upgrading or enrichment treatment before they can be used for ore reduction purposes. Preferably, the upgrading or enrichment of the combustion gases is effected by reducing the CO content by reaction with carbon. Thus, the combustion gases pass from the hearth zone 16 through a line 21 to a gasifier zone 22 to which oxygen and an excess of coal are also supplied through a line 23. The carbon in part of the coal supplied at 23 reacts with the CO in the combustion gases supplied at 21, and the endothermic heat requirements for the reduction reaction are furnished by the sensible heat in the combustion gases supplemented by additional heat evolved in the gasifier 22 by partial combustion of another part of the coal with the oxygen supplied through the line 23. The high oxygen content gas supplied at 23 may be straight commercial grade oxygen or may be an oxygen enriched gas containing at least 85% oxygen just as in the hearth 16. Although reaction of C6 with the carbon content of the coal to produce CC is the primary reduction reaction accomplished in the gasifier zone 22, it will also be appreciated that water vapor contained in the combustion gases fed to the gasifier zone 22 will be reduced by reaction with carbon to form H The high temperature CO-rich gas from the gasifier or reforming zone 22 passes through a line 24 and thence through a heat exchanger 26 where the gases are cooled to a temperature on the order of 500 F. to 600 F. and are thence introduced through a line 27 to a scrubber 23. Water or other liquid cleaning agent is introduced to the scrubber 28 through a line 29 for the purpose of removing ash from the gases, and the efliuent scrubbing liquid is removed through a line 31. The cooled and cleaned reducing gas passes from a line 32 to the inlet of a compressor 33 which discharges through a line 34 into a desulfurization or sulfur removal zone 36, hereinafter described in greater detail. The sulfur-free gas then passes through a line 37 to the heat exchanger 26 wherein the reducing gas is reheated to a suitable temperature of from about 900 F. to about 1800 F. by indirect heat exchange with the hot gases from the gasifier 22. The reheated reducing gas then passes through a line 38 to a pulsating device 39 and thence through the line 12 to the reduction zone 11 as heretofore described. The purpose of the pulsator 39 is to overcome any mechanical bridging or blocking difficulties in the reduction zone 11 which sometimes occurs in connection with non-fluidized moving bed operations.

In order to achieve the balanced operation described in my prior Patent No. 2,919,983, the combustion step in the hearth zone 16 is regulated so that the combustion gas has a CO :CO ratio of from about 0.7 to about 3 with the result that the gas is oxidizing to iron and a high temperature of from about 2900 F. to about 3500" F. is obtained. In addition, the operating conditions in the reforming zone or gasifier 22 must be controlled so that the eflluent reformed gas removed through the line 24 contains not more than about 10% CO with a CO:CO ratio of at least about 7 and a H :CO ratio not greater than about 1. Finally, the operating conditions in the reduction zone 11 are controlled so as to leave a minor portion of unreduced iron oxide having an iron content of from about wt. percent to about 35 wt. percent of the total iron content of the effiuent solids from the reduction zone. When the foregoing critical combination of conditions are observed in connection with the three process stages described briefly above, the desired balanced process is realized, as more fully discussed in my aforementioned prior patent.

In order to avoid sulfur contamination of the molten iron product withdrawn through the line 18 as a result of Wt. percent lime-I-Wt. percent magnesia Wt. percent silica-l-Wt. percent alumina For many iron ores, the acidic gangue constituents of the iron ore are suflicient to provide the desired acidic slag in the hearth zone 16 Without any special steps being taken. However, if such is not the case, extraneous acidic materials such as silica, alumina, or the like must be added at the hearth zone 16 to insure the desired acidic slag. By maintaining a properly acidic slag in the hearth zone, the sulfur introduced with the fuel burned in the hearth zone is retained in the combustion gases, largely as H 5, and leaves the hearth zone through the line 21 without being absorbed in the molten iron withdrawn at 18. Additional sulfur may be incorporated in the gas by reason of the combustion of further quanti ties of fuel in the gasifier or reforming zone 22.

Thus, the cooled and cleaned CO-rich reducing gas which is discharged from the compressor 34 contains substantially all of the sulfur which has been introduced into the system with the fuel burned in the zones 16 and 22. As heretofore explained, it is necessary to remove the sulfur from the reducing gas before the latter is supplied to the reduction zone 11. Otherwise, the sulfur content of the reducing gas would merely be reabsorbed by the solids in the reduction zone 11 and would, therefore, end up in the molten iron. In accordance with the broad principles of the present invention, any convenient technique for sulfur removal may be utilized, and the exact point in the process sequence where the desulfurization step is to be carried out will depend somewhat on the desulfurization method which is selected. In general, it will be most convenient to effect sulfur removal from the gas at the discharge side of the compressor 33 where the gas pressure is high and the gas volume is minimized.

In FIG. 1, the sulfur removal step 35 is shown between the discharge from the compressor 33 and the inlet to the reheating side of the heat exchanger 26. In other words, the sulfur removal in zone 36 is accomplished with cooled gas at a temperature of about 500 F. to 600 F. An example of a suitable sulfur removal process which can be used conveniently at this point in the process sequence is the Girbotol process which comprises scrubbing the gas with an aqueous solution of an ethanolamine. Another sulfur removal technique Which can be conveniently used at the cool side of the heat exchanger 26 comprises passing the sulfur-containing gas stream from the compressor 33 through a bed of sponge iron which will absorb the gaseous sulfur constituents.

In FIG. 2 a modification of the process is shown wherein identical reference numerals are employed to indicate parts of the equipment which are the same as those shown in FIG. 1. In this instance, the sulfur removal step is conducted on the hot side of the heat exchanger 26. Thus, the effluent sulfur-containing gas from the compressor 33 passes through a line 41 to the heat exchanger 26 where the gas is reheated to a temperature of from about 900 F. to about 1800 F., and the reheated gas then passes through a line 42 to a sulfur removal zone 43. The desulfurized gas passes through a line 44 to the pulsator 39 and thence through the line 12 to the reduction zone 11, as previously described. When the sulfur removal step is accomplished at the hot side of the heat exchanger 26, as by means of the sulfur removal zone 43, other types of desulfurization techniques may be more conveniently employed. For example, the hot sulfur-containing gas may be passed through a bed of solid lime-containing material, such as limestone or dolomite, which is effective to remove sulfur at higher temperatures. In addition, a bed of sponge iron may also be used on the hot side of the heat exchanger 26.

As will be evident from the foregoing description, the present invention is not limited to any particular means for elfecting the removal of sulfur from the gas stream. On the contrary, any convenient method may be employed, but the invention affords considerable flexibility in the choice of desulfurizing technique inasmuch as the sulfur removal step may be located at any point in the process sequence between the gasifier 22 and the reduction zone 11. However, for practical reasons, it will generally be most desirable to conduct the sulfur removal operation at the discharge side of the compressor 33 so as to obtain the benefit of maximum pressure. Dependent upon the desulfurization method employed, the sulfur removal step can be carried out on relatively cool gas, as in the FIG. 1 embodiment, or on relatively hot gas, as in the FIG. 2 embodiment. By means of the present invention, a molten iron product having a low sulfur content of from about .03 wt. percent to about .08 wt. percent is readily obtained.

Although the invention has been described with reference to certain specific embodiments by way of illustration, it will be understood that various modifications and equivalents may be resorted to without departing from the scope of the invention as defined in the appended claims.

I claim:

1. In an iron ore reduction process wherein sub-divided iron oxide ore is contacted with a reducing gas in a reduction zone, the resultant reduced solids are transferred to a hearth zone wherein a carbonaceous sulfur-containing fuel is burned to obtain molten iron and slag, the combustion gas from the hearth zone is passed through a treating zone wherein the reducing capacity of the gas is enhanced, and the resultant reducing gas is supplied to said reduction zone; the improvement which comprises maintaining an acidic slag in said hearth zone whereby substantially all of the sulfur from said fuel is retained in said combustion gas and is not absorbed by said molten iron, and desulfurizing said reducing gas from said treating zone before the same is supplied to said reduction zone whereby to avoid sulfur contamination of said reduced solids and thereby insure a low sulfur content in said molten iron.

2. The process of claim 1 further characterized by the steps of supplying reducing gas from said treating zone to a compressor which forces the reducing gas into said reduction zone, and desulfurizing said reducing gas at the discharge side of said compressor.

3. The process of claim 1 further characterized by the steps of passing reducing gas from said treating zone through a cooling zone to a compressor and thence through a reheating zone to said reduction zone, and desulfurizing said reducing gas at the discharge side of said compressor prior to passage of the gas through said reheating zone.

4. The process of claim 3 further characterized in that said desulfurizing step comprises contacting the gas with an aqueous ethanolamine solution.

5. The process of claim 1 further characterized by the steps of passing reducing gas from said treating zone through a cooling zone to a compressor and thence through a reheating zone to said reduction zone, and desulfurizing said reducing gas at the discharge side of said compressor after passage of the gas through said reheating zone.

6. The process of claim 5 further characterized in that said desulfurizing step comprises passing the gas through a bed of solid lime-containing material.

7. In an iron ore reduction process wherein subdivided iron oxide ore is contacted with a reducing gas in a r duction zone, the resultant reduced solids are transferred to a hearth zone wherein a carbonaceous sulfur-containing fuel is burned to obtain molten iron and slag, the combustion gas from the hearth zone is passed through a treating zone wherein the reducing capacity of the gas is enhanced, and the resultant reducing gas is passed successively through a heat exchange zone wherein the gas is cooled, a gas cleaning zone, a compressor, thence through said heat exchange zone wherein the gas is reheated, and thence to said reduction zone; the improvement which comprises maintaining an acidic slag in said hearth zone whereby substantially all of the sulfur from said fuel is retained in said combustion gas and is not absorbed by said molten iron, and desulfurizing said reducing gas at the discharge side of said compressor prior to passage of the gas through said heat exchange zone.

8. The process of claim 7 further characterized in that said desulfurizing step comprises contacting the gas with an aqueous solution of ethanolamine.

9. In an iron ore reduction process wherein subdivided iron oxide ore is contacted with a reducing gas in a reduction zone, the resultant reduced solids are transferred to a hearth zone wherein a carbonaceous sulfur-containing fuel is burned to obtain molten iron and slag, the combustion gas from the hearth Zone is passed through a treating zone wherein the reducing capacity of the gas is enhanced, and the resultant reducing gas is passed successively through a heat exchange zone wherein the gas is cooled, a gas cleaning zone, a compressor, thence through said heat exchange zone wherein the gas is reheated, and thence to said reduction zone; the improvement which comprises maintaining an acidic slag in said hearth zone whereby substantially all of the sulfur from said fuel is retained in said combustion gas and is not absorbed by said molten iron, and desulfurizing said reducing gas after discharge thereof from said heat exchange zone and before the gas is supplied to said reduction zone.

10. The process of claim 9 further characterized in that said desulfurizing step comprises passing the gas through a bed of solid lime-containing material.

11. The process of claim 1 further characterized in that the base to acid ratio of said acidic slag does not exceed about 0.5.

12. The process of claim 7 further characterized in that the base to acid ratio of said acidic slag does not exceed about 0.5.

13. The process of claim 9 further characterized in that the base to acid ratio of said acidic slag does not exceed about 0.5.

References Cited in the file of this patent UNITED STATES PATENTS 1,949,529 Browne Mar. 6, 1934 2,740,706 Paull et al. Apr. 3, 1956 2,755,179 Stalhed July 17, 1956 2,919,983 Halley Jan. 5, 1960 2,928,730 Luerssen Mar. 15, 1960 

1. IN AN IRON ORE REDUCTION PROCESS WHEREIN SUB-DIVIDED IRON OXIDE ORE IS CONTACTED WITH A REDUCING GAS IN A REDUCTION ZONE, THE RESULTANT REDUCED SOLIDS ARE TRANSFERRED TO A HEARTH ZONE WHEREIN A CABONACEOUS SULFUR-CONTAINING FUEL IS BURNED TO OBTAIN MOLTEN IRON AND SLAG, THE COMBUSTION GAS FROM THE HEARTH ZONE IS PASSED THROUGH A TREATING ZONE WHEREIN THE REDUCING CAPACITY OF THE GAS IS ENHANCED, AND THE RESULTANT REDUCING GAS IS SUPPLIED TO SAID REDUCTION ZONE; THE IMPROVEMENT WHICH COMPRISES MAINTAINING AN ACIDIC IN SAID HEARTH ZONE WHEREBY SUBSTANTIALLY ALL OF THE SULFUR FROM SAID FUEL IS RETAINED IN SAID COMBUSTION GAS AND IS NOT ABSORBED BY SAID MOLTEN IRON, AND DESULFURIZING SAID REDUCING GAS FROM SAID TREATING ZONE BEFORE THE SAME IS SUPPLIED TO SAID REDUCTION ZONE WHEREBY TO AVOID SULFUR CONTAMINATION OF SAID REDUCED SOLIDS AND THEREBY INSURE A LOW SULFUR CONTENT IN SAID MOLTEN IRON. 